Subject got me flustered going over English lines (8)
I believe the answer is:
geometry
'subject' is the definition.
(geometry is a kind of subject)
'got me flustered going over english lines' is the wordplay.
'flustered' is an anagram indicator.
'going over' means one lot of letters goes inside another.
'english' becomes 'e' (abbreviation).
'lines' becomes 'ry' (abbreviation for railway).
'got'+'me'='gotme'
'gotme' anagrammed gives 'geomt'.
'geomt' going around 'e' is 'geomet'.
'geomet'+'ry'='GEOMETRY'
(Other definitions for geometry that I've seen before include "Mathematics of shapes and lines" , "branch of mathrmatics" , "Maths dealing with lines, angles etc - Euclid was good at it" , "Branch of mathematics" , "Branch of maths - ask Euclid" .)